use cmake generate expression to simplify some jobs

[WuTUT]

I think there is a simple way to implement these functions

• llvmir_extract_compile_defs_properties
• llvmir_extract_include_dirs_properties
• llvmir_extract_compile_flags
• llvmir_extract_compile_option_properties

if use cmake generate expressions , each function will be only one line.

  set(CMD_ARGS "-emit-llvm" ${IN_STANDARD_FLAGS} ${IN_LANG_FLAGS})
  add_custom_command(OUTPUT ${FULL_OUT_LLVMIR_FILE}
    COMMAND ${LLVMIR_COMPILER} ${CMD_ARGS}
    "$<JOIN:$<TARGET_PROPERTY:${DEPENDS_TRGT},COMPILE_OPTIONS>,;>"
    "$<JOIN:$<TARGET_PROPERTY:${DEPENDS_TRGT},COMPILE_FLAGS>,;>"
    "-D$<JOIN:$<TARGET_PROPERTY:${DEPENDS_TRGT},COMPILE_DEFINITIONS>,;-D>"
    "-I$<JOIN:$<TARGET_PROPERTY:${DEPENDS_TRGT},INCLUDE_DIRECTORIES>,;-I>"
    -c ${ABS_IN_FILE} -o ${FULL_OUT_LLVMIR_FILE}
    DEPENDS ${ABS_IN_FILE}
    IMPLICIT_DEPENDS ${LINKER_LANGUAGE} ${ABS_IN_FILE}
    COMMENT "Generating LLVM bitcode ${OUT_LLVMIR_FILE}"
    COMMAND_EXPAND_LISTS
    VERBATIM)

In addition, there is an important benefit: there is no need to recursively fetch the include directories introduced by target_link_library. For instance, if a library is linked publicly, its interface headers will also be visible. I test in my complicated project and it finds all include paths correctly.

The drawback may be the dependency on a higher version of CMake. However, I haven't tested whether this code works on cmake-version 3.0.

[compor]

Hi there, the support for generator expressions has evolved since before v3.0.
As far as I know, things like TARGET_PROPERTY do not exist in version 3.0 and I'd like to keep this as widely available as possible.

Speed was never a concern.